1. Field of the Invention
The present invention relates to an active electro-optic device, e.g. to an active liquid crystal electro-optic device and, in particular, to a device provided with two thin film gate insulated field effect transistors (hereinafter referred to as TFT) on each picture element, which is named in the present invention as complementary thin film transistor (hereinafter referred to as C/TFTs) having a structure of modified transfer gate(MTG).
2. Description of the Prior Art
An active type liquid crystal electro-optic device utilizing TFT is conventionally known. In this device, an amorphous or polycrystalline semiconductor is used for TFT, while either p-channel or n-channel TFT is used for each picture element thereof. Namely, a N-channel TFT (referred to as a NTFT) is generally linked to the picture element in series. A typical example thereof is indicated in FIG. 1.
Referring to FIG. 1, which shows an equivalent circuit of the liquid crystal electro-optic device in matrix form, NTFT is linked to a liquid crystal 1, in series.
In general, a large matrix device such as 640.times.480, or 1260.times.960 matrix device is used, whereas in the drawing, a simple example of matrix arrangement of 2.times.2 is shown in the same context. Voltage is supplied from peripheral circuits 3, 4 to each picture element. The picture element is turned ON or OFF in accordance with the voltage. When the ON/OFF characteristic of TFT is good, a liquid crystal electro-optic device of high contrast can generally be prepared. When the liquid crystal electro-optic device is actually manufactured, however, there often is a case where the voltage V.sub.LC 5, the output of TFT, i.e. the input for the liquid crystal (referred to as liquid crystal electric potential) does not become "1" (high) when it should have been "1" (high), or otherwise, does not become "0" (low) when it should have been "0" (low). This inconvenience occurs in the case of the TFT which is a switching device applying a signal to the picture element being put into an assymetrical state in the ON/OFF condition.
The liquid crystal 1 is basically insulating, and the liquid crystal electric potential (V.sub.LC) is floating when TFT is OFF. Since the liquid crystal 1 is equivalently a capacitor, the V.sub.LC can be determined based on the charge accumulated therein. When the resistance of the liquid crystal becomes comparatively small at R.sub.LC 6, or current leakage occurs due to the existance of a dust or of an ionic impurity, or when a pin hole is formed in the gate insulating film of the TFT as shown in FIG. 1 at R.sub.GS 7, the charge is leaked therefrom, whereby a V.sub.LC is put into an unstable condition, and high yield cannot be achieved in the liquid crystal electro-optic device that has as much as 200,000-5,000,000 pieces of picture elements in one panel. In particular for the liquid crystal material 1, TN (Twisted Nematic) liquid crystal is typically used. For the orientation of the liquid crystal, a rubbed orientation control film is provided on each electrode. Due to the static electricity generated in the rubbing process, weak dielectric breakdown occurs and leakage occurs between adjacent conductors such as picture element electrodes and electric lines (wires) or leakage occurs in the weak gate insulating film. For the active liquid crystal electro-optic device, it is of particular importance to maintain a certain level of the liquid crystal electric potential, as the same value as an initial value in one frame. Practically, however, this is not always the case because of a lot of defects existing in the TFT part.
In the case where the liquid crystal material is a ferroelectric liquid crystal, it is necessary to increase injection current. For that purpose, TFT is increased in size so as to increase current margin, which is a disadvantage.